Hui Zhang, Fall 2012 1 15-441 Computer Networking Mobility

Hui Zhang, Fall 2012 1

15-441 Computer Networking

Mobility


Many Mobility Scenario’s

Mobile hosts connecting to networked infrastructure (focus of this lecture)

- Cell phones connecting to cell towers

- Laptops connecting to access points

Ad hoc networks: a set of mobile nodes communicating with each other with no additional networked infrastructure

- Extra challenges: routing and possible mobility

Sensor networks (ad hoc +)- More resource constrained mobile nodes

Vehicular networks - Extreme mobility


Routing to Mobile Nodes

Obvious solution: have mobile nodes advertise route to mobile address/32

- Should work!!!

Why is this bad?- Consider forwarding tables on backbone routers

• Would have an entry for each mobile host

• Not very scalable

What are some possible solutions?


How to Handle Mobile Nodes?(Addressing)

Dynamic Host Configuration (DHCP)- Host gets new IP address in new locations

- Problems

• Host does not have constant name/address how do others contact host

• What happens to active transport connections?


How to Handle Mobile Nodes?(Naming)

Naming- Use DHCP and update name-address mapping

whenever host changes address

- Fixes contact problem but not broken transport connections


How to Handle Mobile Nodes? (Transport)

TCP currently uses 4 tuple to describe connection

- <Src Addr, Src port, Dst addr, Dst port>

Modify TCP to allow peer’s address to be changed during connection

Security issues- Can someone easily hijack connection?

Difficult deployment both ends must support mobility


How to Handle Mobile Nodes?(Link Layer)

Link layer mobility- Learning bridges can handle mobility this is how it is

handled at CMU

- Encapsulated PPP (PPTP) Have mobile host act like he is connected to original LAN

• Works for IP AND other network protocols


How to Handle Mobile Nodes?(Routing)

Allow mobile node to keep same address and name How do we deliver IP packets when the endpoint

moves?- Can’t just have nodes advertise route to their

address What about packets from the mobile host?

- Routing not a problem- What source address on packet? this can cause

problems Key design considerations

- Scale- Incremental deployment


Basic Solution to Mobile Routing

Same as other problems in computer science- Add a level of indirection

Keep some part of the network informed about current location

- Need technique to route packets through this location (interception)

Need to forward packets from this location to mobile host (delivery)


Interception

Somewhere along normal forwarding path

- At source

- Any router along path

- Router to home network

- Machine on home network (masquerading as mobile host)

Clever tricks to force packet to particular destination

- “Mobile subnet” – assign mobiles a special address range and have special node advertise route


Delivery

Need to get packet to mobile’s current location Tunnels

- Tunnel endpoint = current location

- Tunnel contents = original packets

Source routing- Loose source route through mobile current location


Mobile IP (RFC 2290)

Interception

- Typically home agent – a host on home network Delivery

- Typically IP-in-IP tunneling

- Endpoint – either temporary mobile address or foreign agent

Terminology

- Mobile host (MH), correspondent host (CH), home agent (HA), foreign agent (FA)

- Care-of-address, home address

Hui Zhang, Fall 2012 13Lecture 21: 2006-11-16 13

Mobile IP (MH at Home)

Mobile Host (MH)

Visiting Location

Home

Internet

Correspondent Host (CH)

Packet


Mobile IP (MH Moving)

Visiting Location

Home

Internet

Correspondent Host (CH)Packet

Home Agent (HA) Mobile Host (MH)I am here


Mobile IP (MH Away – FA)

Visiting Location

Home

Internet

Correspondent Host (CH)

Packet

Home Agent (HA) Foreign Agent (FA)Encapsulated

Mobile Host (MH)


Mobile IP (MH Away - Collocated)

Visiting Location

Home

Internet

Correspondent Host (CH)Packet

Home Agent (HA) Mobile Host (MH)Encapsulated


Other Mobile IP Issues

Route optimality

- Resulting paths can be sub-optimal

- Can be improved with route optimization

• Unsolicited binding cache update to sender Authentication

- Registration messages

- Binding cache updates Must send updates across network

- Handoffs can be slow Problems with basic solution

- Triangle routing

- Reverse path check for security


State of Mobile Networks

Getting a new name and address via DHCP is the most commonly used solution for laptops, as long as

- it is a client machine (what is the difference between a server and client machine?)

- Single consistent TCP connection during mobility is not important

Cell network is the commonly used solution to support continuous mobility

- Overview slides attached. For information only. Not required for exam.

Mobile IP not as widely used- Why?

Will cover ad hoc networks, sensor networks, vehicular networks in two weeks

- Check out Professor Srini Seshan’s 15-446 class next semester


GSM Mobility Management (not required for exam)

Originals by: Rashmi Nigalye, Mouloud Rahmani, Aruna Vegesana, Garima Mittal, Fall 2001

Prof. M. Veeraraghavan, Polytechnic University, New York

• GSM architecture overview– Network layout– Protocols– Addresses & identifiers

• Location management– Call delivery + location update

• Handover management


GSM network layout

GSM Network (PLMN)

MSC region

MSC region

MSC region

Location area

BSC

BSCLocation

area

BTS BTS

PLMN: Public Land Mobile Network

MSC: Mobile Switching Center

BTS: Base Transceiver Station

BSC: Base Station Controller


GSM network layout

BSC

MSC

BSC

BTS

EIRHLR

AUCVLR

BTS

BTS

Um

AAbisE

B,C

OMC

GMSC

PSTN

ISDN


GSM MAP protocol

GSM MAP similar to IS41 MAP MAP uses Transactions Capabilities Part (TCAP)

of the SS7 stack MAP functions:

- Updating of location information in VLRs

- Storing routing information in HLRs

- Updating and supplementing user profiles in HLRs

- Handoff of connections between MSCs


What is a location area (LA)?

A powered-on mobile is informed of an incoming call by a paging message sent over the PAGCH channel of a cell

One extreme is to page every cell in the network for each call - a waste of radio bandwidth

Other extreme is to have a mobile send location updates at the cell level. Paging cut to 1 cell, but large number of location updating messages.

Hence, in GSM, cells are grouped into Location Areas – updates sent only when LA is changed; paging message sent to all cells in last known LA


Addresses and Identifiers

International Mobile Station Equipment Identity (IMEI)- It is similar to a serial number. It is allocated by equipment

manufacturer, registered by network, and stored in EIR International Mobile Subscriber Identity (IMSI)

MCC MNC MSIN

MCC: Country CodeMNC: Mobile Network Code

MSIN: Mobile Subscriber Identification Number

When subscribing for service with a network, subscriber receives (IMSI) and stores it in the SIM (Subscriber Identity Module) card.

The HLR can be identified by a VLR/MSC from the IMSI.



Mobile Subscriber ISDN (MSISDN)- The “real telephone number”: assigned to the SIM

- The SIM can have several MSISDN numbers for selection of different services like voice, data, fax

CC NDC SN

NDC: National Destination Code (NDC identifies operator); SN: Subscriber Number; CC: Country

Code;Digits following NDC identifies the HLR



Mobile Station Roaming Number (MSRN)- It is temporary location dependent ISDN number

- It is assigned by local VLR to each MS in its area.

CC NDC SN


Addresses and identifiers

Temporary Mobile Subscriber Identity (TMSI)- It is an alias of the IMSI and is used in its place for

privacy.

- It is used to avoid sending IMSI on the radio path.

- It is an temporary identity that is allocated to an MS by the VLR at inter-VLR registration, and can be changed by the VLR

- TMSI is stored in MS SIM card and in VLR.


TMSI, IMSI, MSRN and MSISDN

Unlike MSISDN, IMSI is not known to the GSM user. The CC of MSISDN translates to an MCC of IMSI as follows, e.g, Denmark CC: 45 MCC: 238

TMSI is used instead of IMSI during location update to protect privacy. As user moves, TMSI is used to send location update. Thus a third party snooping on the wireless link cannot track a user as he/she moves.

MSRN is the routing number that identifies the current location of the called MS.

- MSRN is temporary network identity assigned to a mobile subscriber.

- MSRN identifies the serving MSC/VLR.

- MSRN is used for call delivery (calls incoming to an MS). MSISDN is the dialed number to reach a GSM user



Location Area ID (LAI)- CC: Country Code, MNC:Mobile Network Code, LAC:

Location Area Code

- LAI is broadcast regularly by Base Station on BCCH

- Each cell is identified uniquely as belonging to an LA by its LAI

CC MNC LAC


Location management

Set of procedures to:- track a mobile user

- find the mobile user to deliver it calls

Current location of MS maintained by 2-level hierarchical strategy with HLRs and VLRs.


Ways to obtain MSRN

1. Obtaining at location update – MSRN for the MS is assigned at the time of each location update, and is stored in the HLR. This way the HLR is in a position to immediately supply the routing info (MSRN) needed to switch a call through to the local MSC.

2. Obtaining on a per call basis – This case requires that the HLR has at least an identification for the currently responsible VLR. When routing info is requested from the HLR, it first has to obtain the MSRN from the VLR. This MSRN is assigned on a per call basis, i.e. each call involves a new MSRN assignment


Routing information: case when MSRN is selected per call by VLR/MSC

If MSRN is allocated to each subscriber visiting at an MSC, then the number of MSRNs required is large. If instead, an MSRN is allocated only when a call is to be established, then the number of MSRNs is roughly equal to number of circuits at MSC – a much smaller number – hence MSRNs typically allocated per call by VLR/MSC

MSISDN

GMSC

HLR

MS

ISD

N

MSC/VLR

MS

RN

IMS

I

MS

RN

MSISDNIMSI, VLR number

MSRN


Call routing to a mobile station: case when HLR returns MSRN

GMSC

BSC

BSC

EIR

HLR

AUCVLR

MSCBTS

BTS

BTS

LA 1

LA 2

ISDN1

MS

1

MSISDN

6

TMSI

4

MSRN

3

MSRN

2

MSISDN

7

TMSI

7

TMSI

7

TMSI

8

TMSI

5

MSRN

MSC


Messages exchanged: call delivery

PSTNGMSC

HLR VLR

Target

MSC

Originating Switch

GMSC HLR VLR

Target MSC

1. ISUP IAM2. MAP_SEND_ROUTING_INFO

3. MAP_PROVIDE_ROAMING_NUMBER

4. MAP_PROVIDE_ROAMING_NUMBER_ack

5. MAP_SEND_ROUTING_INFO_ack

6. ISUP IAM

1

2 3

45

6


Find operation in GSM

ISDN switch recognizes from the MSISDN that the call subscriber is a mobile subscriber. Therefore, forward the call to the GMSC of the home PLMN (Public Land Mobile Network)

GMSC requests the current routing address (MSRN) from the HLR using MAP

By way of MSRN the call is forwarded to the local MSC Local MSC determines the TMSI of the MS (by querying

VLR) and initiates the paging procedure in the relevant LA

After MS responds to the page the connection can be switched through.


Abbreviations

ISC: International switching center OMC: Operations and maintenance center GMSC: Gateway switching center MSC: Mobile switching center VLR: Visitor location register HLR: Home Location register EIR: Equipment Identification register AUC: Authentication center BSC: Base station controller BTS: Base transceiver station MS: Mobile subscriber TMSI: Temporary Mobile Subscriber Identity IMSI: International Mobile Subscriber Identity

Documents

Hui Zhang, Fall 2012 1 15-441 Computer Networking Mobility